The present invention relates to a signal estimator with a phase locked loop, that detects and corrects the phase of a received signal, particularly, deteriorated due to transmission distortion in a digital transmission/communication system in a transmission path. Furthermore, the present invention relates to a memory medium storing programs.
A prior art phase locked loop with an equalizer is disclosed in JP-A-327204/1998. This phase locked loop can correct phase errors even under the environment of a frequency selective path fading in the radio data communication terminal adopting the system of obtaining transmission path characteristics during a preamble period to equalize received signals.
FIG. 14 shows the configuration of a prior art signal estimator with a phase locked loop. Referring to FIG. 14, a delayed decision feedback sequence estimator (DDFSE) 303 estimates a transmission signal sequence with a distorted received signal. In order to create replicas of the received signal, a replica creator 305 convolutes a previously-obtained transmission path impulse response value and a sequence signal estimated by the DDFSE 303.
In this prior art, a phase error signal in a phase locked loop operation is created with the replica signal. This method can subject even received signals with a large transmission distortion to a high-precision phase synchronous correction and operation.
A delay element 304 delays a received signal to compensate the delay caused by the DDFSE 303. Thus, the timing of the output signal of the replica creator 305 can be coincided with that of the output signal of the delay element 304. The phase detector 306 detects the phase difference between the output signal of the replica creator 305 and the output signal of the delay element 304. When the received signal has frequency offset, the phase between transmission and reception changes over time, so that the phase difference obtained by the phase detector 306 varies over time.
Since the transmission path impulse response is constant, the replica creator 305 outputs the received signal with low phase error so long as the DDFSE 303 estimates correctly. However, since the output signal of the delay element 304 varies in phase, the phase detector 306 produces the phase difference. A filter 307 limits the frequency bandwith of the output signal of the phase detector 306 and then outputs the band-limited signal to a voltage-controlled oscillator (VCO) 308. A phase rotator 302 rotates the phase of the received signal with the output signal of the VCO 308. Thus, the phase rotator 302 controls the phase detector 306 to decrease the phase difference signal, so that a phase change due to a frequency offset can be absorbed. The phase rotator 302 outputs the signal of which the phase change has been absorbed.
In the signal estimator with a phase locked loop, according to the prior-art technique, a certain delay occurs till the DDFSE determines an estimated value through the sequence estimation. When a rate in phase change is low, even a large signal estimation delay of the DDFSE enables a normal operation. However, when a rate in phase change is high, the phase locked loop cannot track changes in phase. In the end, the phase is diverged.
In order to improve the phase tracking characteristic of the phase locked loop, there is, for instance, the method of speeding up the response of the phase locked loop by spreading the frequency band of the filter 307 shown in FIG. 14. However, this method tends to be susceptible to external disturbances such as noises so that the precision of tracking phase shifts is deteriorated.
Another countermeasure is to increase the response rate by reducing the estimation delay of the DDFSE. However, since decreasing the delay of the DDFSE deteriorates the estimation capability of the DDFSE, many errors occur in an estimated signal sequence so that the replica signal cannot be normally created. This means that the quality of information on phase differences obtained by the phase detector becomes degraded. Like the former countermeasure, the tracking of the phase locked loop causes disturbances due to noises or distortions contained in the received signal. Finally, the phase locked loop cannot accurately track the phase.
The present invention is made to solve the above-mentioned problems.
The objective of the invention is to provide a system that estimates a received signal subjected to large transmission distortion, using a delayed decision feedback sequence estimator (DDFSE) or maximum likelihood sequence estimator (MLSE).
Another objective of the present invention is to provide a system that can operate with high precision and at high rate a phase locked loop that corrects changes in phase due to frequency offsets, in the above-mentioned estimation system.
The objective of the present invention is achieved by a signal estimator comprising phase rotation means for rotating the phase of a received signal; estimation means for estimating the received signal with an output signal of the phase rotation means, based on a Viterbi algorithm, to obtain an estimation result, and then outputting the estimation result and a minimum error signal used to create a minimum path metric value in an estimation operation; delay means for delaying the output signal of the phase rotation means according to a period during which the estimation means outputs the minimum error signal; phase detection means for detecting a phase shift of the received signal based on the minimum error signal and the delay means; and filter means for limiting the frequency band of an output signal from the phase detection means; and signal generation means for generating a signal of a frequency proportional to the output of the filter means and then controlling the phase rotation means to rotate the phase of the received signal.
Moreover according to the present invention, the signal estimator comprises phase rotation means for rotating the phase of a received signal based on a control signal; a plurality of subtraction means each for obtaining as an error signal a difference between the received signal and plural sample signals; estimation means having means for outputting as a minimum error signal the error signal corresponding to a minimum path, and means for estimating the received signal with an output signal from the phase rotation means, based on a Viterbi algorithm, to obtain an estimation result, and then outputting the estimation result and the minimum error signal; delay means for delaying the output signal from the phase rotation means according to a period during which the estimation means outputs the minimum error signal; phase detection means for detecting a phase shift of the received signal based on the minimum error signal and an output signal from the delay means; filter means for performing a filtering process to limit the frequency band of an output signal from the phase detection means; and signal generation means for generating a signal of a frequency proportional to an output signal from the filter means and then outputting the generated signal as the control signal.
Moreover according to the present invention,
A method for correcting a phase shift of a received signal, comprises the steps of rotating the phase of a received signal based on a control signal; estimating the received signal with the phase-rotated signal, based on a Viterbi algorithm, to obtain an estimation result, and then outputting the estimation result and a minimum error signal used to create a minimum path metric value in an estimation operation; delaying the phase-rotated signal according to a period during which the minimum error signal is output; detecting a phase shift of the received signal based on the minimum error signal and the delayed output signal; limiting the frequency band of the detected signal of which the phase is shifted; and generating a signal of a frequency proportional to a detected signal of which the frequency band is limited, and then outputting the generated signal as the control signal.
Moreover, according to the present invention, in a memory medium wherein a program is stored, the program comprises the steps of rotating the phase of a received signal; estimating the received signal with the phase rotated signal, based on a Viterbi algorithm, to obtain an estimation result, and then outputting the estimation result and a minimum error signal used to create a minimum path metric value in the estimation operation; delaying the phase rotated signal according to a period during which the estimation means outputs the minimum error signal; detecting a phase shift of the received signal based on the minimum error signal and the delay signal; performing a filtering process to limit the frequency band of the phase detected signal; and generating a signal of a frequency proportional to the filtered signal, whereby the signal rotates the phase of the received signal in the phase rotation step, based on the generated signal.
In the signal estimator or the memory medium storing a program, the estimation means may be a maximum likelihood sequence estimator (MLSE) or a delayed decision feedback sequence estimator (DDFSE). Moreover, the estimation means may have a plurality of subtraction functions each for obtaining as an error signal a difference between the received signal and plural sampling signals; and a function for outputting as the error signal the error signal corresponding to a minimum path.